The present invention relates to a method for estimating the position and trajectory of a moving source of seismic waves. In particular, it relates to positioning of a drill during drilling of boreholes in rock.
Methods for estimating the position or trajectory of a drill during drilling of boreholes in rock are known, which are based on widely varying techniques, such as compass based instruments, gyroscope based instruments and instruments estimating the position of the drill bit by detecting seismic waves emitted at the position of the drill bit. Of these, several commercialised compass based instruments have failed to achieve their goal due to magnetic reasons, and gyroscope based instruments have failed to achieve their goal due to drifting problems and fragility. Several seismic wave based instruments have failed to achieve the required accuracy due to factors such as varying wave propagation speed in the ground which arises when the ground has an inhomogeneous distribution of rock types with different specific propagation speed, erroneous position estimations when seismic waves are reflected from cracks and at interfaces between rocks having different wave propagation properties.
U.S. Pat. No. 5,680,906 discloses a method for location of the position and trajectory of a drill during drilling of boreholes in rock. The method is based on detecting the seismic waves emitted by the drill bit on impact, and takes advantage of the fact that the transient waves emitted at each impact, hereinbelow termed event, are very similar to each other, such that seismic data recorded at geophones can be averaged for a number of events. The averaging of data from several similar events give a increased signal-to-noise level, which can be useful when detecting events from a drill bit positioned farther away from the geophones than would otherwise be possible. If the precision in determining the arrival time for the events is noise limited, this could also lead to an increased precision.
A general problem with prior art methods for location of a drill, using seismic wave based instruments, is their inherent disability to discern arrival time differences due to varying propagation speeds for waves propagating through rocks with unknown and varying wave propagation properties. If, for example, the wave propagation speed in the rock on a first side of the drill bit is higher than on the other side, the calculated position of the drill bit will be shifted from its real position towards the first side. This is because the geophones situated on the first side will detect a shorter arrival time for an event, than the geophones situated on the other side, and this is interpreted as corresponding distances being shorter and longer than their true values, respectively. Earth models, which make certain assumptions regarding the distribution of the wave propagation properties of the rock, can improve the estimation of the position of a seismic event, but with no advance knowledge of the actual wave propagation properties of the rock, seismic wave based instruments will suffer from a position accuracy which is comparatively low.
It is an object of the present invention to provide a method for estimating the position and trajectory of a moving source of seismic waves, which suffers significantly less from the above mentioned problem.
The object is achieved with a method according to claim 1.